The invention relates to a record carrier containing information in an optically readable information structure comprising information areas which are arranged in information tracks and, which alternate with intermediate areas in the track direction. The invention also relates to an apparatus for reading such a record carrier.
In the case of a disk-shaped record carrier the information tracks may be arranged in concentric tracks, or, alternatively, interconnected quasi-concentric tracks to form a spiral track.
Such a record carrier and apparatus are known, inter alia from Netherlands Patent Application No. 78 06378, which has been laid open to public inspection. The known record carrier may contain a television program, the information being encoded in the frequency and/or the dimensions of the information areas in the track direction. The information areas may comprise pits pressed into the record carrier surface or hills projecting from said surface. Alternatively, the information may be encoded in digital form, the information areas and intermediate areas also having a constant dimension in the track direction. A specific combination of information areas and intermediate areas then represents a specific combination of digital zeros and ones.
In such optical record carriers it is desirable to maximize the information density and, hence, to maximize the playing time, for example, of a recorded television program. For this purpose the tracks would have to be arranged as close as possible to each other. However, the distance between the information tracks cannot be arbitrarily small. For known record carriers in which the information areas of adjacent information tracks have the same geometry, except for the dimension in the track direction, all the information areas will influence the radiation of the read beam in the same manner. The read spot formed on the information structure by the read beam is a diffraction-limited radiation spot with a specific, for example Gaussian, intensity distribution. The half-value diameter of this spot, i.e. the distance between two points in the spot where the intensity is 1/e.sup.2 of the intensity in the centre of the spot, is of the order of the track width. This means that even in the case of correct tracking the read spot will illuminate regions adjacent the track being read, and some of the radiation may even be incident on adjacent tracks. The amount of radiation on the adjacent tracks increases as the track distance decreases. A specific portion of the radiation which is incident on the adjacent tracks and which is modulated by the information areas of these tracks can reach the radiation-sensitive detector, which is intended to receive the radiation which has been modulated by the track to be read. This effect, the crosstalk effect, determines the minimum distance between the information tracks.
In Netherlands Patent Application No. 78 06378, which has been laid open to public inspection, it is proposed to increase the information density by giving the information pits of the adjacent tracks different depths and by reading said information tracks with beams of different wavelengths. The depths and wavelengths are selected so that the information areas of a first information track produce a maximum modulation in a beam of a first wavelength, while the information pits of adjacent second information tracks hardly influence the beam, i.e. are hardly observed by this beam. The last-mentioned pits do produce a maximum modulation in a beam of a second wavelength, which beam in turn is hardly influenced by the information pits of the first information tracks. The information tracks may then be arranged substantially closer to each other, without excessive crosstalk.
However, this proposal presents some practical problems. First of all, two radiation sources are required for producing the two beams of different wavelengths, so that the read apparatus becomes more intricate. Secondly, in order to obtain adequate separation during read-out of the two types of information pits, comparatively deep pits, of the order of a several times the wavelength of the associated read beam, have to be formed with an accuracy of the order of a tenth of the wavelength of the read beam. This is a technologically difficult problem.